SiC fibers are known in the art for providing mechanical strength at high temperatures to fibrous products, such as high temperature insulation, belting, gaskets, or curtains, or as reinforcements in plastic, ceramic, or metal matrices of high performance composite materials. To provide good mechanical strength to these products or materials, the SiC fibers have a relatively high density (i.e., low residual porosity) and fine grain sizes. However, producing SiC fibers with these properties is difficult because the SiC fibers typically undergo coarsening or growth of crystallites and pores during a high-temperature heat treatment.
Sintering aids have been used to improve densification of the SiC fibers and to prevent coarsening, allowing the SiC fibers to be fabricated with high density and fine grain sizes. These sintering aids are typically compounds of boron. As disclosed in U.S. Pat. No. 5,366,943 to Lipowitz et al., which is incorporated by reference in its entirety herein, the SiC fibers are formed by converting amorphous ceramic fibers to polycrystalline SiC fibers. The ceramic fibers are heated in the presence of a sintering aid to produce the polycrystalline SiC fibers. The sintering aid is boron or a boron-containing compound, such as a boron oxide (“B2O3”). An example of SiC fibers prepared by this process is SYLRAMIC®, which is available from COI Ceramics, Inc. (San Diego, Calif., an affiliate of ATK Space Systems).
Another method of forming SiC fibers is by spinning a polycarbosilane resin into green fibers, treating the green fibers with boron, and curing and pyrolyzing the green fibers, as disclosed in U.S. Pat. No. 5,071,600 to Deleeuw et al. Other methods of forming SiC fibers are known, such as spinning organosilicon polymers into fibers, curing the fibers, and ceramifying the fibers at elevated temperatures. However, many of these methods undesirably introduce oxygen or nitrogen into the SiC fibers. When these SiC fibers are heated to temperatures above 1400° C., the oxygen or nitrogen is volatilized, causing weight loss, porosity, and decreased tensile strength in the SiC fibers. In addition to SiC fibers, SiC bodies are known to be formed by molding SiC powder and elemental carbon into a desired shape and heating the molded structure in a boron-containing environment.
While many methods of producing SiC fibers (or SiC bodies) are known, components or products formed from conventional SiC fibers, such as those described above, are not suitable for use in nuclear applications due to the boron compound used as the sintering aid. The boron compound typically includes boron-10 (“10B”). Boron has thirteen isotopes, two of which, 10B and 11B, are naturally occurring. The natural abundance of 10B and 11B is 19.9% and 80.1%, respectively. However, 10B is the most commercially available isotope because 10B is more easily extracted from ore than 11B. 10B absorbs neutrons and is used in control rods of nuclear reactors, as a shield against nuclear radiation, and in instruments for detecting neutrons. However, 10B is unstable and undergoes fission when irradiated, producing a gamma ray, an alpha particle, and a lithium ion. Therefore, when a component formed from conventional SiC fibers is irradiated, the boron compound undergoes fission, which is accompanied by outgassing and degradation of the SiC fibers or the SiC bodies. As such, conventional SiC fibers are not suitable for use in a component to be used in the nuclear industry, such as in a nuclear reactor.
It would be desirable to produce SiC fibers or SiC bodies that are more stable to irradiation for use in components to be used in the nuclear industry. For instance, it would be desirable to produce SiC fibers or SiC bodies that are useful in nuclear applications without outgassing or degradation.